An electrochromic element is a material that can change color according to the supply of electricity, and is applicable to a smart window or an information display device, etc.
Among them, the smart window generally uses a thin film composed of a tungsten oxide so that it shows blue color or becomes colorless according to the direction of applying voltage. That is, if negative voltage is applied, the smart window has deep blue color to reduce the transmission of incident light. If applied with positive voltage, the smart window becomes colorless so that more light can be incident through the window. Herein, the time to change color takes from a millisecond to a few minutes, and light transparency is generally 1 to 30%, which is greatly different from that of transparent glass ranging from 40 to 80%. Since the building using the smart window is thermally insulated, it has excellent ability of saving energy, which is consumed in heating, air conditioning and lighting.
The key point of the electrochromic element for use in the smart window is the fact that the color has to be easily changed according to the supply of electricity. Further, the electrochromic element should have the flexibility so that it can be responsive to the bending operation thereof. Such a property is an important factor required for attaching the electrochromic element to the surface of glass, plastic or metal, or for fabricating the same into a thin film. As the electrochromic element, a polymer material is proper for the reasons that while it has poor electric conductivity than metal, it can be fabricated through compounding, it is lighter than metal, and it has ductility.
Referring to FIG. 1, the monomer constituting the polymer material may have electric conductivity. The electrically conductive monomer has a double bond therein. The double bond consists of π-bond and σ-bond, in which the π-bond is accompanied with many electrons, so that certain electrons can migrate in accordance with the n-bond. Accordingly, the n-bond can provide the polymer with electrical characteristic. Attention had been riveted, as the electrochromic element, to polyacetylene, that has the double bond while having such a molecule structure, because it has the electric conductivity similar to Cu (approximately 106S/cm). However, due to its oxidizing characteristic in air, it was fatally destitute of stability so that its value was admitted as only academic achievement. Then, naturally, the study for electrically conductive polymer was focused upon various candidate polymer materials capable of securing physical, chemical stabilities, and attention was drawn to polyaniline, that have been known since 140 years ago.
Among polymer materials, the polyaniline is easy to compound, and is stable under room temperature and atmospheric pressure. Further, the aniline, the unit of the polyaniline, includes a benzene ring, and may have an oxidizing state in multi-stage due to the double bond and the resonance structure of the benzene ring. Furthermore, based on respective oxidizing states, the electric conductivity can be widely varied from a nonconductor to a conductor, so that it has the electrochromic characteristic according to the oxidizing state.
For the reason above, the polyaniline has been widely applied to various fields, such as electrochromic material, electronic material, thin film, lithography, catalyst, sensor, nano fiber (particle), thin film transistor and super capacitor. Further, the polyaniline replaced an existing inorganic metal material with an organic material, so that it had the usefulness as a next generation high-tech material such as the development of a flexible display.
Nevertheless of such an advantage, however, the polyaniline had a problem in that it was hardly applicable to an electrochromic element or a display device, which would require the ductility, because it was transformed into excessively stable material after being compounded. Further, it is difficult to fabricate the polyaniline in a thin film because the compounded polyaniline is hardly dissolved in most of organic solvents as well as is very brittle and easily broken. In order to overcome the above problems and fabricate the polyaniline in a thin film to thereby use it as an electrochromic element, a study has been started to manufacture a polyaniline thin film.
As a method of manufacturing polyaniline in a thin film, there were chemical polymerization, electrochemical polymerization, dispersion polymerization, and copolymerization. The chemical polymerization is conducted such that aniline monomer and a polymerization agent (generally, ammonium persulfate, (NH4)2S2O8)) are mixed into an aqueous solution, that is adjusted to acidity, to start polymerization, and then a substrate is immersed in the solution to thereby polymerize it as well as to form, on the surface thereof, a polyaniline thin film. This method is the simplest method among the methods of manufacturing the polyaniline thin film.
However, the thin film obtained from this method is not easy to adjust the thickness or the adhesion force thereof. Further, the chemical polymerization is not easy to adjust the characteristic of the thin film as compared to the other methods. Moreover, since only a part of the polyaniline participates in forming the thin film, most of polyaniline remains in the solution. The remaining polyaniline cannot be used any more in the formation of the thin film, so that too much polyaniline is used wastefully rather than that used in the formation of the thin film. This is problematic. Furthermore, the remaining polyaniline is carcinogen that is harmful to the health of a human being.
Another method for the formation of the thin film is the electrochemical polymerization. The basic process of this method is the same as that of the chemical polymerization. However, in this method, a certain electric potential is applied in order to control the characteristic of the thin film, so that this method can advantageously control the adhesion force, the density, the electrical property, and others of the polyaniline thin film. However, this method also has the same problem as the chemical polymerization.
Still another method for the formation of the polyaniline thin film is the dispersion polymerization. The key point of this method is that while once the polyaniline is prepared by the polymerization, it is uniformly dispersed in form of very small particles in a solution. When the polyaniline is dispersed as such, it has an effect as if it is dissolved. That is, such an effect is similar to the formation of the polyaniline solution, so that the dispersing solution is applied to various types of substrates to thereby fabricate a thin film. Further, using this method, a thin film can be fabricated relatively stably. However, while the polyaniline thin film applicable to an electrochromic element needs to maintain sufficient adhesion force for increase in lifetime and reliability of the element, the thin film obtained by this method has a problem in that the adhesion force is insufficient so that the thin film is easily detached from the element.
Yet still another method for the formation of the polyaniline thin film is the copolymerization of the polyaniline. That is, polymer and copolymer that are easily dissolved in organic solvent are formed so that the polyaniline that is not dissolved is to be dissolved. Using such a method, the polyaniline thin film can be easily obtained. However, nevertheless the formation of the polyaniline thin film is ultimately for using the electric property of the polyaniline, if another type of polymer (most case, such polymer is greatly different in electrical property from the polyaniline) is mixed therein, a problem is caused in that the electrical property of the polyaniline thin film is deteriorated or lost.